This invention relates generally to a grooved endoprosthesis, and more specifically to the use of a Particular arrangement and type of groove pattern to promote biological fixation of endoprostheses of various types. A Particular application of the groove pattern of the present invention, as used on a femoral stem of a hip joint prosthesis, is also disclosed.
Component loosening is one of the primary reasons for failure of cemented hip arthroplasties. In the past decade, attempts have been made to correct this problem by providing for biological fixation of the components. A common technique involves use of a porous coating on the surface of the component to encourage bone ingrowth for fixation purposes. A commonly used porous surface for cementless fixation is sintered (cobalt-chromium). This surface is produced by the high temperature sintering of spherical alloy beads to a similar alloy substrate. The process forms metallurgical bonds between the spherical beads and the underlying substrate. Other implants employ surfaces of plasma-sprayed titanium. This surface is produced by spraying heated powdered titanium alloy onto a titanium alloy substrate. Unlike the sintered Co-Cr surface, which has relatively well-defined pores, the titanium plasma spray coating forms a somewhat irregular porous surface which provides channels for bone ingrowth, but which lacks a three-dimensional interconnecting system.
The process of manufacturing either of these porous coated surfaces is difficult and expensive. Strict quality control requirements result in only a percentage of the implants actually produced by either process being acceptable for clinical use. There is also considerable concern about the negative effect which the sintering process may have on the crystalline structure of the underlying substrate.
Another problem which has been observed in the use of known hip joint systems relates to the proper distribution of stresses within the prosthesis and throughout the surrounding bone. If too little stress is applied to the bone, resorption can occur leading to atrophy of the affected area. Too much stress may also lead to resorption and atrophy, or may result in undesirable hypertrophy of the affected area. In some prior art femoral stem designs, too much force is transmitted through the relatively rigid stem to the distal portion, resulting in hypertrophy of the bone surrounding the distal portion, and atrophy of the bone surrounding the proximal portion of the stem. Accordingly, there exists a need for an improved hip joint prosthesis which addresses these and other problems of prior hip joint designs.
An object of the present invention is to provide an improved arrangement of grooves on an implantable prosthesis which has been found to be especially effective in promoting biological fixation of the prosthesis through the mechanism of bone ingrowth.
Another object of the present invention is to provide a femoral stem for a hip joint prosthesis which provides for relatively strong fixation of the proximal end of the stem to the femur, and which provides for relatively weak fixation of the distal end of the stem.
Another object of the present invention is to provide a femoral stem for a hip joint prosthesis which has precisely controlled mechanical and metallurgical properties, and which provides a precisely dimensioned surface for bone fixation.
Yet another object of the present invention is to provide a femoral stem for a hip joint prosthesis which is relatively easy and inexpensive to manufacture.
Still another object of the present invention is to provide a femoral stem for a hip joint prosthesis which can be more readily removed from the bone in the event resection becomes necessary.
These and other objects are attained in a hip joint prosthesis which includes a femoral stem which has a head and an elongated shank. The head is adapted to cooperate with either a natural acetabulum or a prosthetic acetabulum. The shank has a proximal end adjacent the head and a distal end which extends longitudinally into the intramedullary canal of the femur. The shank further has anterior (front), posterior (back), lateral (outside), and medial (inside) surfaces. A plurality of grooves extending generally transversely to the longitudinal axis of the shank are formed on proximal portions of selected surfaces of the shank, while distal portions of these selected surfaces are left relatively smooth. In a preferred embodiment, these grooves are substantially parallel grooves formed in the anterior and posterior surfaces of the shank. The grooves on the anterior and posterior surfaces are preferably oriented approximately perpendicularly to a line of force transmitted from the head to the shank of the femoral stem. The grooves preferably lie in planes which intersect the longitudinal axis of the shank at an angle of approximately 60.degree.-80.degree., as measured from a point on the axis which lies above the proximal end of the shank along an arc which extends toward the lateral surface of the shank, with an especially preferred orientation of the grooves lying in planes which intersect the longitudinal axis at an angle of approximately 70.degree. as measured from a point on the axis which lies above the proximal end of the shank along an arc which extends toward the lateral surface of the shank.
An especially preferred embodiment of the invention includes a plurality of substantially parallel grooves in the proximal portion of the medial surface. These grooves preferably extend generally longitudinally along the proximal portion of the medial surface, and are especially effective in providing resistance to torque or rotational forces on the stem.
The grooves formed in the anterior, posterior and medial surfaces are substantially equal in depth and width, and are substantially uniformly spaced apart by a distance which is preferably equal to the depth and width of the grooves. The width of the grooves ranges from approximately 0.014" to 0.021" (350 microns to 535 microns), with an especially preferred width being approximately 0.0197" (500 microns). The preferred depth of the grooves ranges from approximately 0.015" to 0.025" (380 microns to 635 microns). Grooves having these dimensions in width, depth and spacing have been found to be particularly effective in promoting biological fixation of endoprostheses by tissue ingrowth. Such grooves are well-suited for use with the femoral stem described in detail in this application, and for other types of implantable prostheses as well. In the femoral stem described below, approximately 25 grooves are spaced along the proximal 1/4-1/3 of the anterior and posterior surfaces of the shank, while approximately three grooves are formed in the proximal portion of the medial surface.
The present invention provides for a macro-interlocking or fixation of the prosthesis to the surrounding bone. The grooved surface of the prosthesis can be precisely manufactured without adversely affecting the properties of the substrate material. The machining process is repeatable and relatively efficient, resulting in a decrease in both expense and difficulty in the manufacture of the prosthesis when compared to several prior art devices.
The femoral stem prosthesis of the present invention provides strong proximal fixation of the stem to better and more naturally distribute stresses throughout the femur. Prior art designs which allow for fixation of the stem throughout substantially all or most of its length provide for the transmission of too much stress through the stem to the distal portion of the shank. This lead to atrophy in the proximal portion of the femur, and undesired hypertrophy in the area of the femur surrounding the distal portion of the stem. The stem of the present invention distributes the stresses in the proximal portion of the femur, preventing resorption and atrophy which may otherwise result. In the event resection of the stem is necessary, the design of the present invention allows easier access to the interlocked area, and will result in the disturbance of a smaller portion of the femur than might otherwise occur in the case of a distally interlocked shank.